Appreciable amounts of sulfur dioxide are contained in many industrial gases vented into the atmosphere from plants involved in roasting, smelting and sintering sulfide ores such as chalcopyrite (CuFeS.sub.2), iron pyrites (FeS.sub.2), and pyrrhotite (FeS), in stack gases from power plants burning sulfur-bearing coal, or in exit gases from other industrial operations involving the combustion of sulfur bearing fuels, such as fuel oil. Air pollution as the result of sulfur dioxide emission in these gases presents not only a health hazard, but also results in loss of valuable sulfur values. Hence, sulfur dioxide is desirably recovered from such gases, desirably in the form of elemental sulfur.
The reduction of sulfur dioxide, including its reduction to elemental sulfur, has been investigated extensively, and there has been a myriad of references published on this subject. For instance, in U.S. Pat. Nos. 2,270,427; 2,388,259 and 2,431,236 the reduction of sulfur dioxide with natural gas, such as methane, is described wherein the sulfur values are recovered in an essentially three step reaction. In the first step the reduction of sulfur dioxide contained in the off gases from copper smelting operations is reacted with methane at temperatures of approximately 2280.degree. to 2360.degree. F. with a refractory material performing as a surface catalyst. The chief sulfur-containing by-products were carbonyl sulfide and hydrogen sulfide. The carbonyl sulfide is then reacted with additional sulfur dioxide at temperatures of about 800.degree. to 840.degree. F. over a bauxite catalyst to produce sulfur, and the hydrogen sulfide is reacted with still further amounts of sulfur dioxide at a temperature of about 410.degree. to 450.degree. F. in the presence of bauxite to produce sulfur by the well-known Claus reaction.
Similarly, in U.S. Pat. No. 3,199,955 to West et al. a process is disclosed employing three catalytic reduction steps to convert the sulfur dioxide to elemental sulfur, wherein the last step involves the well-known Claus reaction. In the first step, the reduction of sulfur dioxide with methane is achieved at temperatures of 1470.degree. to 1830.degree. F. in the presence of a catalyst including activated alumina, bauxite, calcium sulfide and quartz. With this process it is reported that between about 40 and 60% of the inlet sulfur dioxide appears in the production gases from the first step as elemental gaseous sulfur, the remainder is found as hydrogen sulfide, carbonyl sulfide, carbon disulfide and sulfur dioxide. The second and third steps of this process are essentially the same as those reported in the earlier patents. The carbonyl sulfide and carbon disulfide are reacted with sulfur dioxide to produce sulfur at a temperature of about 735.degree. F. in the presence of a suitable catalyst, such as alumina; and in the final step (Claus reaction) the hydrogen sulfide is reacted with sulfur dioxide at a temperature of about 390.degree. to 530.degree. F. in the presence of a catalyst, such as activated alumina, to produce additional sulfur. While the process of that patent is said to be applicable to sulfur dioxide-containing gas streams of high concentration, such as pure or concentrated sulfur dioxide, use of concentrated sulfur dioxide in the particular process configurations disclosed by West et al. will tend to pose difficult problems of temperature control within the reactors employed, as a result of which attainment and maintenance of stable operating conditions becomes difficult or impossible. This is so because gas streams containing relatively large proportions of sulfur dioxide on reduction thereof generate proportionately larger amounts of heat concentrated in relatively smaller volume, and the reduction reaction, once initiated, tends to proceed with considerable speed to the point of becoming uncontrollable.
Desirably, reduction of sulfur dioxide with a reducing agent should produce as few undesirable by-products as possible of those which may be formed, such as carbon monoxide, carbon disulfide, carbonyl sulfide and hydrogen, thereby achieving efficient utilization of the reducing agent. By-product formation depends upon a number of variables, including temperature, flow rate of reactants, ratio of reactants employed, and type of catalyst selected. Advantageously, equilibrium is sought because at equilibrium conditions the products formed in the reaction and their proportions are predictable, and under these circumstances conditions can predictably be chosen which favor reduction of sulfur dioxide to elemental sulfur, rather than to other reduction products. For instance, under specific equilibrium conditions at preferred operating temperatures, as hereinafter defined, employing methane as the reducing gas, the methane can be completely reacted with the sulfur dioxide so that the process can be expressed by the equations: EQU 2SO.sub.2 + CH.sub.4 .fwdarw. S.sub.2 +2H.sub.2 O+CO.sub.2 EQU 6so.sub.2 + 4ch.sub.4 .fwdarw. 4co.sub.2 +4h.sub.2 o+4h.sub.2 s+s.sub.2
furthermore, essentially no detectable amounts of carbonyl sulfide and/or carbon disulfide are formed in the reduction of sulfur dioxide under equilibrium conditions at preferred operating temperatures and employment of reductant of essentially stoichiometric proportions. Accordingly, when chemical equilibrium is achieved under the above conditions there is no need to provide additional equipment to convert these by-products to additional sulfur and there is no loss of unreacted methane. Ideally, the reduction of sulfur dioxide with a reducing gas is conducted under conditions which favor equilibrium at lowest possible temperatures.
Yushkevich et al., Zh. Khim. Prom, No. 2, p. 33-37 (1934) reported on a study of reduction of sulfur dioxide with methane and disclosed that equilibrium of the reduction of sulfur dioxide with methane can be achieved under certain conditions within temperature range of from approximately 700.degree. to 1000.degree. C., and space velocities within the range of from 70 to 1000. Yushkevich et al. concluded from the results of their experiments that equilibrium is achieved in the reduction of SO.sub.2 with methane at temperatures of 800.degree. to 1000.degree. C., by maintaining the space velocity of the gas reactants (sulfur dioxide and reducing agent) through the catalyst bed in the order of up to about 500. However, Yushkevich et al. found that at temperatures of 900.degree. and 1000.degree. C., respectively, and space velocities of 1000 (equivalent to a contact time of 0.8 second), the reaction product contained 2.1% and 0.7% methane, respectively. Yushkevich et al. report that at 800.degree. C. and space velocities as low as 200 (equivalent to a contact time of about 4 seconds), substantial amounts of unreacted methane remain in the product gas mixture.
As can be appreciated, when the space velocity of the reactants which enter into the reaction is decreased (i.e, contact times increased) in order to achieve equilibrium, larger size process equipment would have to be employed for the same amount of gas treated thereby substantially increasing the capital cost of a commercial plant.
See, also, Averbukh et al., Khim. Prom., (3), 200 (1971), describing reduction of sulfur dioxide with methane or natural gas at temperatures of 750.degree. to 900.degree. C. in the presence of catalyst such as aluminum oxide, alumina, bauxite, alunite and dunite, employing sulfur dioxide-to-methane ratio of 1.54:1 at gas velocities chosen to obtain reaction times in the order of 0.07 to 0.48 seconds. Further, Averbukh and coworkers have investigated and reported on the kinetics of the thermal reduction of concentrated sulfur dioxide-containing gases by methane, employing gases containing 10, 20, 30, 40 and 100% SO.sub.2 in their experiments (Averbukh et al., Khim. Prom., (44), 753 (1968).
Copending application of A. W. Michener et al., Ser. No. 238,644 filed March 27, 1972 discloses a process for reduction of sulfur dioxide wherein equilibrium may be achieved at temperatures from 1000.degree. to 2400.degree. F. employing extremely short contact times and very high velocities of the gases through the catalyst bed, under substantially complete consumption of the reducing agent.
U.S. Pat. No. 3,653,833 to Watson et al. describes a method for reducing sulfur dioxide to elemental sulfur and/or other gaseous sulfur compounds with a reducing gas in the presence of a catalyst at temperature within the range of from 1000.degree. to 2400.degree. F. by passing a gaseous reaction mixture of sulfur dioxide-containing gas with the reducing gas serially first through a regenerative heat exchanger to raise the temperature of the gas mixture to 1000.degree. to 2400.degree. F., then passing the heated gas mixture through a reaction chamber containing a catalyst to obtain a product gas stream comprising hydrogen sulfide, sulfur dioxide and sulfur, and finally passing the product gas stream through a second regenerative heat exchanger to absorb heat therefrom to reduce the temperature of that stream to about 700.degree. to 800.degree. F. In that process the regenerative heat exchangers are subjected to continuously alternating heat absorbing cycles while maintaining the passage of the gaseous reaction mixture through the reaction chamber in the same direction always. The exothermic heat of reaction is utilized in the regenerative heat exchanger system to preheat the feed gases, since the extremely corrosive nature of the gaseous product at high temperatures make use of conventional shell and tube exchangers difficult or impossible. The regenerative heat exchangers alternately preheat and cool the gases as the flow direction is periodically changed. The flow through the reactor is always in the same direction, and always in series with both regenerative heat exchangers.
While the process described by Watson et al. represents a significant advance in the art and is capable of handling sulfur dioxide-containing gas streams of widely varying sulfur dioxide content, utilization therein of gas streams containing high concentrations of sulfur dioxide tends to pose difficult problems of temperature control because of the relatively greater heat release during sulfur dioxide reduction in a relatively smaller volume of gas, leading to extremely high temperatures in the reaction zone and relatively short cycle durations in the regenerative heat exchangers, among others.
U.S. Pat. No. 3,928,547 to W. D. Daley et al. discloses a process for reduction of sulfur dioxide to elemental sulfur wherein a mixture of sulfur-dioxide-containing gas and a hydrocarbon reductant is reacted at elevated temperature in the presence of minor amounts of elemental sulfur, resulting in lowered initiation temperatures for the reduction reaction and in moderation of the progress of the reaction, thereby avoiding violent temperature rise.
More stringent pollution controls have in the recent past been imposed on coal-burning power plants with respect to both particulate as well as sulfur dioxide emissions. However, stack gases of coal-burning power plants generally contain less than about 1 percent, and more likely less than about 1/2 of 1 percent by volume of sulfur dioxide. Processing of the diluted sulfur dioxide contained in such stack gases to elemental sulfur is considered uneconomical unless the sulfur dioxide prior to reduction can be concentrated. There are available a number of sulfur dioxide recovery processes wherein sulfur dioxide is recovered from stack gases and obtained in more concentrated form as gas, generally containing more than about 8 percent by volume sulfur dioxide and ranging upwards in sulfur dioxide concentration up to 100 percent by volume, dry basis. Typical of these recovery processes are the so-called "regenerative alkaline" processes, wherein an alkaline agent such as sodium sulfite, ammonium sulfite, alkali or alkaline earth metal carbonate or magnesium oxide (MgO) strip the sulfur dioxide from the flue gas by combining chemically with the sulfur dioxide. In a separate regeneration step the agent is reconstituted and the sulfur dioxide gas is recovered. Other processes available include the so-called "regenerative solid adsorption" processes wherein a sulfur adsorber, such as activated char or activated carbon adsorbs the sulfur dioxide and then the sulfur dioxide is desorbed to produce a sulfur dioxide gas stream. Also, there are available the so-called "regenerative organic" processes which differ from the alkaline regenerative absorption processes in that an organic absorbing medium is employed. All of these regenerative processes, however, produce a sulfur dioxide-containing gas stream of high sulfur dioxide content of up to 100 percent sulfur dioxide by volume, dry basis.
It is an object of the present invention to provide an improved process for reducing sulfur dioxide in a sulfur dioxide-containing gas stream with a gaseous reducing agent in the presence of a catalyst, which is particularly suited to utilize a sulfur dioxide-containing gas stream of high sulfur dioxide concentration, such as may be obtained from a regenerative sulfur dioxide absorption process.